1. Field of the Invention
The present invention relates to a magneto-optical recording medium and a reproducing method thereon. In particular, the present invention relates to a magneto-optical recording medium and a reproducing method thereon which are suitable for high density recording and which make it possible to perform reproduction by magnifying a minute recording magnetic domain which is extremely smaller than a reproducing light spot.
2. Description of Related Art
The magneto-optical recording medium is a highly reliable recording medium having a large storage capacity on which information is rewritable. Therefore, the magneto-optical recording medium begins to be practically used as a computer memory and the like. However, a technique for performing recording and reproduction at a higher density is demanded in view of the increase in amount of information and the advance of the apparatus to acquire a compact size. In order to record information on the magneto-optical recording medium, a recording system based on the magnetic field modulation is used, in which a magnetic field having a polarity corresponding to a recording signal is applied to a portion at which the temperature is raised, while irradiating the magneto-optical recording medium with a laser beam. This system makes it possible to perform overwrite recording, in which high density recording has been achieved. For example, recording has been achieved with a shortest mark length of 0.15 .mu.m. A recording system based on the optical modulation has been also practically used, in which recording is performed by radiating a power-modulated light beam corresponding to a recording signal while applying a constant magnetic field.
When it is intended to reproduce information from a recording mark having been recorded at a high density, a problem arises concerning the optical reproducing resolving power which is determined by a spot diameter of a reproducing light beam. For example, it is impossible to perform reproduction while distinguishing a minute mark having a magnetic domain length of 0.15 .mu.m by using a reproducing light beam having a spot diameter of 1 .mu.m. In order to eliminate the restriction for the reproducing resolving power resulting from the optical spot diameter of the reproducing light beam as described above, one approach has been suggested concerning the magnetically induced super resolution technique (MSR) as described, for example, in Journal of Magnetic Society of Japan, Vol. 17, Supplement No. S1, p. 201 (1993). This technique utilizes the occurrence of the temperature distribution over a magnetic film included in a reproducing light beam spot when a magneto-optical recording medium is irradiated with a reproducing light beam. A magnetic mask is generated in the spot so that the effective spot diameter, which contributes to signal reproduction, is reduced. The use of this technique makes it possible to improve the reproducing resolving power without reducing the actual spot diameter of the reproducing light beam. However, in the case of this technique, since the effective spot diameter is decreased by means of the magnetic mask, the amount of light which contributes to the reproduction output is decreased, and the reproduction C/N is lowered to that extent. As a result, it is difficult to obtain sufficient C/N.
Japanese Laid-Open Patent Publication No. 1-143041 discloses a method for performing reproduction on a magneto-optical recording medium comprising a first magnetic film, a second magnetic film, and a third magnetic film which are magnetically coupled to one another at room temperature. Assuming that the first, second, and third magnetic films have Curie temperatures of T.sub.C1, T.sub.C.sub.2, and T.sub.C3 respectively, there are given T.sub.C2 &gt;room temperature and T.sub.C2 &lt;T.sub.C1, T.sub.C3. The coercive force H.sub.C1 of the first magnetic film is sufficiently small in the vicinity of the Curie temperature T.sub.C2 of the second magnetic film. The coercive force H.sub.C3 of the third magnetic film is sufficiently larger than a required magnetic field in a temperature range from room temperature to a required temperature T.sub.PB which is higher than T.sub.C2. The magneto-optical recording medium is used to perform reproduction while magnifying the recording magnetic domain in the first magnetic film. This method utilizes the increase in temperature of the medium when the reproducing light beam is radiated so that the magnetic coupling between the first and third magnetic films is intercepted. In this state, the magnetic domain in the first magnetic film is magnified by using the externally applied magnetic field and the diamagnetic field acting on the recording magnetic domain. It is noted that this technique uses the second magnetic film in which the Curie temperature is set to be lower than the temperature of the readout portion during reproduction. However, the present invention does not use any magnetic film having such a magnetic characteristic.
Japanese Laid-Open Patent Publication No. 8-7350 discloses a magneto-optical recording medium comprising a reproducing layer and a recording layer on a substrate, on which reproduction can be performed while magnifying a magnetic domain in the recording layer during the reproduction. When the magneto-optical recording medium is subjected to reproduction, an alternating magnetic field is used as a reproducing magnetic field to alternately apply a magnetic field in a direction to magnify the magnetic domain and a magnetic field in the opposite direction. Thus, the magnetic domain is magnified and reduced for each of the magnetic domains.
The present inventors have disclosed, in International Publication WO 97/22969, a method for performing reproduction on a magneto-optical recording medium, in which a reproducing light beam is radiated onto the magneto-optical recording medium having a magneto-optical recording film which is a perpendicularly magnetizable film at a temperature not less than room temperature to detect the magnitude of the magneto-optical effect so that a recorded signal is reproduced.
The magneto-optical recording medium to be used is a magneto-optical recording medium comprising, on the magneto-optical recording film, an auxiliary magnetic film which causes transition from an in-plane magnetizable film to a perpendicularly magnetizable film when the temperature exceeds a critical temperature, with a non-magnetic film interposed therebetween. The magneto-optical recording film and the auxiliary magnetic film satisfy a relationship of room temperature&lt;T.sub.CR &lt;T.sub.CO, T.sub.C provided that the magneto-optical recording film and the auxiliary magnetic film have Curie temperatures of T.sub.CO and T.sub.C respectively, and the critical temperature of the auxiliary magnetic film is T.sub.CR. The recording signal is reproduced by irradiating the magneto-optical recording medium with the reproducing light beam which is power-modulated at the same cycle as that of a reproducing clock or at a cycle created by the multiplication of an integer and the reproduction clock. In this reproducing method having the foregoing feature, the reproducing light beam is modulated to have reproducing light powers of Pr.sub.1 and Pr.sub.2 at the same cycle as that of the reproducing clock or at the cycle created by the multiplication of an integer and the reproducing clock. This patent document discloses that one of the reproducing light powers of Pr.sub.1 and Pr.sub.2 is a power to cause magnification of the magnetic domain in the auxiliary magnetic film. The principle of the reproducing method will be explained by using a schematic diagram concerning the reproducing method shown in FIG. 19. In this reproducing method, as conceptually shown in FIGS. 6A and 6B, a magneto-optical recording medium is used, which has a structure comprising, on a recording layer 10, an auxiliary magnetic layer 8 with a non-magnetic layer 9 intervening therebetween. At first, a predetermined recording pattern as shown in FIG. 19(a) is recorded on the second type magneto-optical recording medium as the magneto-optical recording medium by using, for example, the optical modulation recording system. In FIG. 19(a), the recording mark is recorded at a shortest mark pitch DP, and the recording mark length DL is set to give DL=DP/2. Upon reproduction, a pulse laser beam, which is modulated to have two kinds of reproducing powers Pr2, Pr1, is used as the reproducing laser beam to be radiated so that the cycle which synchronized with the recording mark position is DP, and the light emission width of the high power Pr2 is DL as shown in FIG. 19(b). The light beam having the low reproducing power Pr1 is always radiated in an erasing state (onto portions at which no recording mark exists), and the light beam having the high reproducing power Pr2 is radiated in a recording state (onto portions at which the recording mark exists) and in the erasing state.
FIG. 19(c) illustrate a reproduced signal waveform obtained by radiating the reproducing pulse laser as shown in FIG. 19(b). On the other hand, FIG. 19(d) illustrates a reproduced signal waveform obtained when the same track is subjected to reproduction by using a continuous light beam having a constant reproducing light power. Pr2 and Pr1 are selected as follows. That is, Pr2 is a recording power to cause the magnification of the magnetic domain in the auxiliary magnetic film 8 as described later on. Pr1 is a power to extinguish the magnified magnetic domain. When the reproducing power is selected as described above, the amplitude H.sub.p1, which is provided between the recording state and the erasing state observed during the reproduction with the pulse light beam, is allowed to satisfy H.sub.p1 &gt;H.sub.dc with respect to the amplitude H.sub.dc obtained upon the reproduction with the constant laser beam. Further, the magnetization information, which is recorded in each of the magnetic domains of the magneto-optical recording film, can be independently magnified and reproduced without being affected by adjacent magnetic domains.